The profession of fighting fires includes numerous inherent dangers. Beyond the obvious risks associated with getting burned by a fire's flames, firefighters also face many other safety hazards associated with fire when entering a burning structure. One of the essential fuels necessary for fire and the combustion of materials is oxygen. As a fire burns and spreads throughout a structure its flames continuously deplete the atmospheric oxygen inside the structure. As the fire burns hotter, it can cause oxygen levels within the structure to drop to dangerously low levels. During the early stages of a fire there is usually more oxygen available than the fire can consume. The fire's burn rate will typically slow down as the levels of oxygen inside the structure are depleted. The fire can only consume oxygen at the rate at which additional oxygen can enter the area in which the fire is burning. This phenomenon causes incomplete combustion, which results in the production of toxin laden smoke comprised of deadly particles, vapors and gases. In addition to the structure itself, its contents will also burn during a fire. Various toxins are released into the structure's atmosphere as the structure and its contents burn. Depending on the structure's composition and contents, the levels of toxins released into the atmosphere can be significantly high. Some toxins that are produced during combustion include carbon monoxide, hydrogen chloride, hydrogen cyanide, carbon dioxide, nitrogen dioxide, phosgene, ammonia and chlorine. The structure's rooms will often fill with the toxic smoke making it nearly impossible for anyone inside the structure to breath. The low concentrations of oxygen and high concentrations of smoke within a structure during a fire make for significantly difficult and hazardous conditions that firefighters need to overcome in order to stay within the structure long enough to rescue victims trapped inside and extinguish the fire.
A backpack respirator, also known as a Self Contained Breathing Apparatus (SCBA), is an essential piece of equipment used by firefighters. SCBAs allow firefighters to breath while they are inside a burning structure thereby permitting them to remain within the hostile environment for longer durations.
The SCBAs are typically comprised of a frame, a harness system that straps to the fireman's shoulders, back and waist, an air cylinder and valve connected to a faceplate-mounted regulator and a face mask with head harness. The frame and harness system support an inverted cylinder filled with oxygen on the fireman's back so that the cylinder's valve assembly faces downward when the cylinder tank is installed on the SCBA's frame. Some SCBAs are configured so as to support two cylinders side-by-side, thereby increasing the SCBA's maximum potential runtime.
The oxygen cylinders used with SCBAs have housings that are typically constructed from aluminum. The aluminum housing is often wrapped with a fiber composite to help further strengthen and protect the cylinder. The cylinder housing has a base at its distal end and a neck at its proximal end. The housing's base can be either flat, about flat or it can be rounded/spherical. At the proximal end of the cylinder housing, fixedly or removably attached to the housing's neck, is the cylinder's valve assembly. The valve assembly usually threads into the cylinder housing's neck.
The cylinder valve assembly is comprised of several components, including a cylinder hanger bracket or cylinder collar bracket. Cylinder hanger brackets (or cylinder collar brackets) are configured so as to be secured to cylinders in different ways. Some cylinder brackets are secured to the cylinder using screws or bolts which pass through mounting holes on the cylinder bracket and thread directly into either the cylinder's valve assembly or housing. Other cylinder brackets include a round aperture at the end opposite the bracket slot where the cylinder's neck and valve assembly pass through. The cylinder bracket is then firmly pressed in place upon the cylinder's neck by the cylinder's valve assembly.
Regardless of how a cylinder bracket attaches to the cylinder, one element common to all cylinder brackets is an aperture near one of the cylinder bracket's ends known as a bracket slot. The bracket slot is always positioned away from the cylinder when the cylinder bracket is installed upon the cylinder and is used when securing the cylinder to a SCBA. The cylinder bracket is used to connect the cylinder to the SCBA's frame assembly and support it on the SCBA. The SCBA frame and harness typically includes a hook at the base of the frame designed to engage the bracket slot on the cylinder bracket. The SCBA's hook serves the dual purpose of keeping the cylinder in place within the frame and partially supporting the weight of the cylinder on the fireman's back. Additional harnesses and/or fasteners on the SCBA wrap around the cylinder's housing to securely fasten the cylinder in place within the SCBA's frame.
The oxygen cylinders used with SCBAs are categorized according to their duration ratings. Some standard cylinders include the 30, 45 and 60 minute sizes. The length/height of the different cylinders is approximately the same although their diameters will vary according to their rated durations. The cylinders with higher duration ratings generally have larger diameters than cylinders with lower duration ratings. Although their ratings and diameters may vary, the bracket slots of cylinder brackets found on cylinders with different ratings still remain the same, allowing cylinders with different ratings to be used interchangeably with the same SCBA.
The different duration ratings of cylinders are determined by a National Institute for Occupational Safety and Health (NIOSH) Breathing Machine Test which simulates an average adult at a moderate work rate of 40 liters per minute. The duration ratings are only estimated durations of how long the cylinders are expected to deliver oxygen to the respirator when the respirator is in use. The actual durations may vary considerably depending on several factors, including the physical condition of the user, the user's degree of physical activity, the degree of training or experience the user has with the particular respirator, whether or not the cylinder is fully charged at the start of the work period, the atmospheric pressure in the area in which the respirator is being used, the fit of the faceplate on the user and on the condition of the SCBA. Typically, oxygen cylinders will last for shorter durations and will need to be replaced more often when firemen are more active, when the fire is hotter and when the level of toxins in the atmosphere is high. This is primarily caused because the fireman's level of oxygen demand and consumption will be significantly higher under such conditions. Accordingly, firemen often need to replace their oxygen cylinders while fighting a single fire. Especially before entering a larger structure, such a multi-floor building, or when faced with a more severe fire, firefighters will often carry additional oxygen cylinders into a building with them to avoid having to exit the structure and return to the fire truck for replacement cylinders in the midst of fighting the fire. In addition to using oxygen cylinders with their SCBAs, in certain instances fireman also need to carry extra cylinders for use with various air-driven rescue tools such as air lifting bags, and air driven impact tools used to break through concrete and stone. Further, in addition to carrying spare oxygen cylinders, firefighters also need to carry other rescue tools such as halligan bars, axes, picks, flashlights, and wire cutters when entering a burning structure.
Firefighters usually carry spare cylinders in their hands by grasping the cylinder from its valve assembly. In doing so, they are usually limited to carrying one cylinder in each hand, assuming each hand is free, which is most often not the case do to the need to carry other equipment. Some firefighters, depending on their size and strength, try grasping two cylinders in each hand although this is often very difficult and unstable, particularly when travelling up and down a burning building's stairwell while also trying to carry additional equipment. Still others attempt to carry two replacement cylinders by using a rope tied to the valve assembly of each cylinder and then draping the rope with the suspended cylinders over their shoulder(s). Again, this is a very unstable method of transporting cylinders because the cylinders can swing uncontrollably as the fireman maneuvers through the burning structure. The swinging cylinders can strike other rescuers, and can cause the fireman carrying the cylinders to lose his/her balance due to the shifting load. Another issue with using a rope to carry cylinders is that with wear, the rope can deteriorate causing it to eventually break or snap. This is especially likely to occur when the rope is in use under the stress of the weight. A rope loaded with cylinders that breaks unexpectedly while a fireman is transporting the cylinders can cause the cylinders to fall and possibly hurt the fireman and/or anyone else within his/her general vicinity. Yet another major problem with this method of transporting cylinders includes the added time necessary to fasten and unfasten the rope to each of the cylinders by tying the rope to the cylinder valves. The time it takes to tie and undue the knots in the rope will ultimately result in a slower response time to secure and replace cylinders on the SCBA. The time can be even longer in the event the fireman is unable to untie the knots in the rope. The added time spent fumbling with the rope can have deadly consequences for the fireman whose original cylinder is completely depleted and who is trying to install a new cylinder before running out of breath. The need to wear gloves during a fire also makes tying and untying knots difficult.
Oxygen cylinders are typically stored horizontally in both the firehouse and on fire trucks using special cylinder racks. Firefighters will routinely remove cylinders from the racks one at a time when they need to be used, or serviced. When the firefighters have finished using or servicing the cylinders they will then replace them back upon the racks, again one cylinder at a time. Having to remove and replace cylinders from the racks one at a time can be a time consuming process. Performing the task at a quicker rate may require the involvement of additional man power. In the event of an emergency the time it takes to perform this task can be critical. Furthermore, devoting additional people to accomplish the task during an emergency, when they can be better utilized somewhere else, can be detrimental to the firefighters' rescue efforts.